1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus and, more particularly, to a photoelectric conversion apparatus for removing noise included in a signal photoelectrically converted by a photoelectric conversion element.
2. Related Background Art
In a conventional photoelectric conversion apparatus, when a signal photoelectrically converted by a photoelectric conversion element is to be read out, an unnecessary component such as a dark signal or a drive noise from the photoelectric conversion element is often included in and output together with the readout signal. The dark signal is a dark current of a photosensor, and the drive noise is noise generated when the photosensor is driven to read out a signal.
In order to remove these noise components from a photoelectric conversion element, the assignee of the present invention proposed a photoelectric conversion apparatus having the following arrangement in Japanese Patent Application No. 62-279390.
FIG. 1 is a partial circuit diagram showing an arrangement of the above photoelectric conversion apparatus.
Referring to FIG. 1, noise from a photoelectric conversion element is stored in a capacitor C.sub.1 via a MOS transistor Qh.sub.1, and a signal from the photoelectric conversion element is stored in a capacitor C.sub.2 via a MOS transistor Qh.sub.2.
The stored noise and signal are sequentially transferred, via a buffer amplifier Qh.sub.5 and a MOS transistor Qh.sub.9, to an output signal line AL by MOS transistors Qh.sub.3 and Qh.sub.4 controlled by pulses .phi..sub.A and .phi..sub.B, respectively. Thereafter, the noise is removed from the signal. Since a signal and noise are passed through the same amplifier Qh.sub.5 to remove the noise such as fixed pattern noise from the signal, noise from the photoelectric conversion element and offset noise generated by the amplifier can be simultaneously removed.
The assignee of the present invention also proposed a photoelectric conversion apparatus having the following arrangement in Japanese Patent Application No. 63-72112.
FIG. 2 is a partial circuit diagram showing an arrangement of the above photoelectric conversion apparatus.
First, as shown in FIG. 2, MOS transistors Qh and Qhr are turned on, one terminal of a capacitor C.sub.1 is grounded, and a MOS transistor Q.sub.VC is turned off, thereby floating the emitter of a photoelectric conversion element S. As a result, noise from the photoelectric conversion element is stored in the capacitor C.sub.1 and a capacitor C.sub.V. Assume that a potential obtained in this state is Vn. A signal is stored in the base of the photoelectric conversion element S, and a pulse .phi..sub.VC during a storage period is raised to clear the capacitor C.sub.V. At this time, the potential changes from Vn to GND. If the transistors Qh and Qhr are kept OFF, the potential at the output side of the capacitor C.sub.1 is -Vn.
The signal from the photoelectric conversion element S is transferred to and stored in the capacitor C.sub.V. At this time, the potential rises from GND by Vs. Meanwhile, the potential at the output side of the capacitor C.sub.1 rises from -Vn by Vs.
By turning on the transistor Qh, a signal from which noise components are removed can be output.
Of the above two photoelectric conversion apparatuses, however, the former photoelectric conversion apparatus must comprise a high-precision and high-speed clamp circuit and a sample-and-hold circuit as external circuits in order to remove noise from the photoelectric conversion element and offset noise from a point sequential signal. In addition, it is difficult to apply an IC arrangement to the entire apparatus. Furthermore, in order to incorporate these circuits in a single chip, positive and negative pulses must be used for the clamp or the sample-and-hold circuit.
In the latter photoelectric conversion apparatus, when an output signal is passed through the buffer amplifier, offset noise generated by the amplifier is superposed on the signal.